The present invention concerns a dewatering arrangement in the press section of a web-forming machine of the type having a press suction roll which is arranged to form a press nip with another roll in the web-forming machine, and a press fabric which is arranged to run via the press suction roll, and having a dewatering saveall extending essentially over the entire width of the web-forming machine. The saveall being installed in an area between an opening gap and a closing gap defined by the press suction roll and the press fabric, and having drainage facilities for removing the collected water from the saveall.
In a web-forming machine, the press suction roll serves two main purposes. In addition to dewatering taking place in the press nip, the press suction roll controls the travel of the web on the press section. However, the shell of the press suction roll is open, and a vacuum effect is only exerted on a part of the circumference of the shell in order to establish a vacuum zone. The bores and grooves on the shell hence contain much water after the suction zone, and this water is ejected to the environment of the press suction roll after the press nip for example by centrifugal force. Water removal is only partial, and some water still remains in the bores of the shell and returns to the press nip. This impairs the dewatering ability of the press section and disturbs pressing.
In order to prevent the spreading of water and to remove water from the press suction roll, web-forming machines usually have a dewatering arrangement which contains a saveall. The saveall follows the shape of the press suction roll and extends over a part of the length of the circumference of the shell. The saveall is also often provided with ventilation, which aims to create a vacuum in the saveall. In practice, water released from the surface of the shell is ejected against the walls of the saveall and descends to the bottom part of the saveall and finally runs out of the press section via a discharge connection. The saveall can also be provided with doctoring equipment which doctors water from the surface of the shell into the saveall. Despite ventilation, washing showers and doctoring, some water still remains in the bores of the press suction roll, and the water, in turn, contains fibers and impurities. Furthermore, a vacuum remains inside the press suction roll even after the press nip, and the vacuum partially prevents the removal of water from the bores. Fibers and impurities hence gradually accumulate on the walls of the bores, finally clogging the bores entirely. The fouling of the press suction roll impairs the dewatering ability of the press section, complicates web control and usually leads to a premature replacement of the press suction roll for service purposes.
Savealls of known type have a flat and open structure, and they also have many types of reinforcements to accomplish sufficient rigidity. Especially internal reinforcements inside savealls form corners and pockets which disturb and decelerate the flow rate of water-containing air inside the saveall. A washing shower pipe used in known solutions, installed in conjunction with the press suction roll, also has the same effect. Furthermore, the flow inside the saveall is turbulent. Impurities and fibers contained in water-containing air hence accumulate on the inner surfaces of the saveall, on the top surface of the washing shower pipe and in the above-mentioned corners and pockets. In this way, the saveall becomes gradually dirty, and if the dirt accumulations become loose, they cause quality deviations and even web breaks in production. The press section fabrics and roll coatings may also be damaged.
The saveall covers the circumference of the shell only partially so that some water mist spreads to the environment of the press suction roll. Furthermore, there are uncontrolled circulating air flows, which contain much water mist and steam, in the opening gap formed by the press fabric and the press suction roll after the press nip. The impurities contained in the air flows escaping from the saveall also become attached onto other rolls and the frame structures of the press section. In this way, for example access bridges become slippery and favorable conditions are created for bacterial growth. In order to avoid accidents and production interruptions, both the saveall and its environment require regular cleaning, which calls for a shutdown because of washing.